1. Field of the Invention
The present invention concerns optical fibers and, more particularly, a method for manufacturing an optical fiber, whereby the fiber is drawn from one end of a preform heated to the glass softening temperature, while a gas flows around the preform and its flow direction corresponds to the fiber drawing direction.
2. Description of the Prior Art
The direct manufacture of an optical fiber having the required fiber profile from different types of glass is not possible because of the small outside dimensions of the optical fiber. The manufacture of an optical fiber is therefore based on a preform with a diameter in the centimeter range, which can be produced in several different ways, e.g., with multimode-gradients or single mode fibers, depending on the type of optical fiber desired. One method of producing a preform is the chemical vapor deposition method (CVD-method) which involves applying a core glass material to the inner surfaces of a quartz glass tube by chemical deposition from the vapor phase of the core glass material onto the inside surfaces of the quartz glass tube, which is subsequently collapsed into a preform. The Outside Vapor Deposition method (OVD-method) involves the outside coating of a glass core with a glass material by deposition from the vapor phase of the glass material on the outside of the glass core. The Vapor Axial Deposition method (VAD-method) provides for the axial coating of a glass core with a glass material by deposition from the vapor phase of the glass material axially along the glass core.
Glass is an amorphous material, which can be brought to a low viscosity condition by heating. With the quartz glass normally used today to manufacture fibers, a noticeable softening of the glass occurs at a glass softening temperature of about 1900 to 2200 K. Glass softening at the glass softening temperature is used to draw a preform into a thin glass fiber. To that end, the preform is heated in a drawing oven, which can be a graphite resistance oven, e.g., wherein a heating element under protective gas is brought to the glass softening temperature by pulsating direct or alternating current, or an induction oven, in which a tube, e.g., made of zirconium oxide or graphite, is brought to the glass softening temperature by an electromagnetic field.
The chimney effect in the circular or tube-shaped internal space of such known drawing ovens produces an air or gas flow through the inside of the oven when at the operating temperatures required for fiber manufacture. After the preform is introduced into the oven, the local conditions inside the oven lead to turbulence in this air or gas flow, which in turn has detrimental effects on the fiber damping and the fiber strength. Such turbulence still exists when a protective gas is blown from the top through the inside of the drawing oven. The turbulence of the protective gas results from the temperature of the protective gas increasing from an upper inlet area of the oven to about the middle of a drawing area, i.e., the production area of the oven containing the heated end of the preform where the fiber is pulled or drawn, and decreasing again in an outlet area. At the same time, an increase in the cross section of the area adjacent to the heated end of the preform takes place in the drawing direction of the fiber in an annular gap between the preform surface and the internal oven wall. The increased cross-sectional area of the annular gap results from the reduced cross-section of the preform as it is drawn down into a fiber. The increased cross-sectional area of the annular gap causes a change in the thus far mostly laminar flow of the gas being introduced from above, resulting in a turbulent flow due to the decrease of the flow speed, precisely in this sensitive fiber drawing area. Particles which may be carried in with the mostly laminar flow are deposited on the heated end of the preform or on the fiber being pulled from it. Such particles are the cause of later fiber breakage or possible defects in the glass surface or the fiber surface.